Characterizing white LEDs for general illumination applicati

[Watson A.Name \"Watt Sun]

I agree with their findings, especially page 4, paragraph 5 which gives
white LEDs a much shorter lifetime rating than the generally asserted
100,000 hours. The white LEDs I put on a PC board before xmas and left
running continuously have dimmed to being all but worthless as illumination.

http://www.lrc.rpi.edu/programs/solidstate/pdf/SPIE3938-39_Narendran.pdf

 

[Myron Samila]

"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com> wrote in message
news:c4kdn3$6i9sg$1@hades.csu.net...

I agree with their findings, especially page 4, paragraph 5 which gives
white LEDs a much shorter lifetime rating than the generally asserted
100,000 hours. The white LEDs I put on a PC board before xmas and left
running continuously have dimmed to being all but worthless as illumination.

http://www.lrc.rpi.edu/programs/solidstate/pdf/SPIE3938-39_Narendran.pdf

Where they of good quality? Osram claims 30,000 hours from their white LEDs. They do
last longer, but the white component only lasts 30k hours.

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[Watson A.Name - \"Watt Su]

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:qRJbc.31538$j57.1436862@news20.bellglobal.com...

"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com
wrote in message
news:c4kdn3$6i9sg$1@hades.csu.net...
I agree with their findings, especially page 4, paragraph 5 which
gives
white LEDs a much shorter lifetime rating than the generally
asserted
100,000 hours. The white LEDs I put on a PC board before xmas and
left
running continuously have dimmed to being all but worthless as
illumination.


http://www.lrc.rpi.edu/programs/solidstate/pdf/SPIE3938-39_Narendran.pdf

Where they of good quality? Osram claims 30,000 hours from their
white LEDs. They do
last longer, but the white component only lasts 30k hours.

Yeah, that's the problem. The phosphor dims, just like the phosphor in
a CRT when you have a pattern constantly on the screen in the same
place. The bright spots get burned in, so you can read it even when you
turn the monitor off. The same thing happens to the phosphor in the
LED, it dims, and then there's little blue and almost no white.

THe LEDs I used were from Hong Kong, and no, they were cheap, so gauging
by the price they were low quality.

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

In sci.electronics.misc "Watson A.Name - \"Watt Sun, the Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

| Yeah, that's the problem. The phosphor dims, just like the phosphor in
| a CRT when you have a pattern constantly on the screen in the same
| place. The bright spots get burned in, so you can read it even when you
| turn the monitor off. The same thing happens to the phosphor in the
| LED, it dims, and then there's little blue and almost no white.
|
| THe LEDs I used were from Hong Kong, and no, they were cheap, so gauging
| by the price they were low quality.

There are better phosphors these days that don't have that burn in problem.
I guess cheaper LEDs don't use those.

However, I don't like the spectra I get from a CRT screen for illumination
purposes. If the same spectra comes from while LED's I won't like them,
either. There are LEDs that produce single discrete colors from all over
the spectrum. Maybe the right phosphor mix can be made to do that for one
single LED, too. A CRT would not use such a mix because it needs to have
discrete colors. But a white LED in theory could, and could potentially
emulate an incandescent light source very well. A pair of them could be
used to balance the color temperature, too. Otherwise I guess I'll have
to make a box with lots of different LEDs all over the spectrum, and set
their levels to get the proper broad spectrum of light output as white.

--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------

 

[D. Weatherwax]

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

Thanks for the info.


"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com> wrote
in message news:106vk2tvmckbde@corp.supernews.com...

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:qRJbc.31538$j57.1436862@news20.bellglobal.com...
"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com
wrote in message
news:c4kdn3$6i9sg$1@hades.csu.net...
I agree with their findings, especially page 4, paragraph 5 which
gives
white LEDs a much shorter lifetime rating than the generally
asserted
100,000 hours. The white LEDs I put on a PC board before xmas and
left
running continuously have dimmed to being all but worthless as
illumination.


http://www.lrc.rpi.edu/programs/solidstate/pdf/SPIE3938-39_Narendran.pdf

Where they of good quality? Osram claims 30,000 hours from their
white LEDs. They do
last longer, but the white component only lasts 30k hours.

Yeah, that's the problem. The phosphor dims, just like the phosphor in
a CRT when you have a pattern constantly on the screen in the same
place. The bright spots get burned in, so you can read it even when you
turn the monitor off. The same thing happens to the phosphor in the
LED, it dims, and then there's little blue and almost no white.

THe LEDs I used were from Hong Kong, and no, they were cheap, so gauging
by the price they were low quality.

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[Spehro Pefhany]

On Sun, 4 Apr 2004 17:34:09 -0500, the renowned "D. Weatherwax"
<davew@joink.com> wrote:

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

Thanks for the info.

Most "white" LEDs actually emit blue light from the junction. The red
and green is generated from the blue light by phosphor in the plastic.
Red + green + blue makes white in additive colors (as on a CRT
screen). It's also possible to generate light that appears white by
using red, green and blue LED chips, but that is a lot more expensive.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[John Fields]

On Sun, 4 Apr 2004 17:34:09 -0500, "D. Weatherwax" <davew@joink.com>
wrote:

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

---
The white LED's I have on hand have a water-clear plastic T1-3/4
housing, and when you look through it into the reflector cup (when
it's not illluminated) you see a light yellow non-reflective coating
on it, which glows bright white when the LED is on. The chip itself
doesn't seem to be emitting blue, so I believe that what's happening
is that it's generating UV which is being used to pump the blend of
phosphors in the cup into emitting the three primary colors which we
then perceive as white light.

--
John Fields

 

[Dave Platt]

Is this true? Do white LED's actually use phosphor to generate the white
light?

Yup... not unlike a standard fluorescent tube.

I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's.

"White" isn't a color, but a combination thereof (as you realize), so
you'd need several different dopings or junction chemistries in order
to generate the necessary wavelengths. As far as I know, nobody's
managed to do this sort of multiple implanting within a single
junction, or on multiple closely-located junctions on a single die.

There are tricolor LED assemblies available, which have separate
red/green/blue LED chips mounted on a single substrate, usually
feeding into a single lens assembly. They're more complicated to
drive, due to the fact that the different wavelengths require
different bandgaps, which require different forward voltages - you
can't simply wire the three chips in parallel, or the red one (which
has the lowest forward voltage) will hog all of the current. They're
also expensive.

Fun, though, since you can generate a wide gamut of colors by driving
the three chips to different brightness levels (adjusting the current,
or using PWM). You'll sometimes find these used in small
consumer-electronics components, as indicators - the Yaesu VX-7
handheld amateur radio has a multicolor indicator which I believe is
powered by one of these.

If phosphor
generates the white light, what wavelength excites the phosphor? It can't be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

Last I heard, they were using a blue LED, and a phosphor which absorbs
some of the blue and then emits yellow and a smaller amount of red.

Yes, you can get ultraviolet LEDs these days.

--
Dave Platt <dplatt@radagast.org> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Watson A.Name - \"Watt Su]

"Dave Platt" <dplatt@radagast.org> wrote in message
news:1071a4isitsh8f5@corp.supernews.com...

Is this true? Do white LED's actually use phosphor to generate the
white
light?

Yup... not unlike a standard fluorescent tube.

Yeah, it's not uncommon (sic) to see this stumbling block to easy
reading nowadays. I would say "Yup... like a standard fluorescent
tube."

I'm not trying to pick on Dave, it's a journalistic trend that seems to
be "not uncommon" nowadays. I'm ashamed to see, above, that I've fallen
into its trap..

I assumed it was doping in the silicon junction that
produced
specific wavelengths of light just like red or green LED's.

"White" isn't a color, but a combination thereof (as you realize), so
you'd need several different dopings or junction chemistries in order
to generate the necessary wavelengths. As far as I know, nobody's
managed to do this sort of multiple implanting within a single
junction, or on multiple closely-located junctions on a single die.

There are tricolor LED assemblies available, which have separate
red/green/blue LED chips mounted on a single substrate, usually
feeding into a single lens assembly. They're more complicated to
drive, due to the fact that the different wavelengths require
different bandgaps, which require different forward voltages - you
can't simply wire the three chips in parallel, or the red one (which
has the lowest forward voltage) will hog all of the current. They're
also expensive.

Fun, though, since you can generate a wide gamut of colors by driving
the three chips to different brightness levels (adjusting the current,
or using PWM). You'll sometimes find these used in small
consumer-electronics components, as indicators - the Yaesu VX-7
handheld amateur radio has a multicolor indicator which I believe is
powered by one of these.

If phosphor
generates the white light, what wavelength excites the phosphor? It
can't be
infrared (can it??). Do they make LED's that output in the UV region,
and
then put phosphor in front of the junction?

Last I heard, they were using a blue LED, and a phosphor which absorbs
some of the blue and then emits yellow and a smaller amount of red.

Yes, you can get ultraviolet LEDs these days.

--
Dave Platt <dplatt@radagast.org
AE6EO
Hosting the Jade Warrior home page:
http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[zapbuzz]

ok, if you have them running on say a 50mhz signal pulse generated
then they will last longer.
I recon make sure you only apply enough energy to power them up not the
max rated tollerance.

 

[repatch]

Yes it is phosphor. most "white" LEDs are actually Blue LEDs with phosphor.
They are trying to get UV LEDs working well enough to use for white LEDs
since the result is better light, but I don't think they have solved all the
problems yet. TTYL

"D. Weatherwax" <davew@joink.com> wrote in message
news:c4q2h60q0t@enews3.newsguy.com...

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't
be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

Thanks for the info.


"Watson A.Name - "Watt Sun, the Dark Remover"" <NOSPAM@dslextreme.com
wrote
in message news:106vk2tvmckbde@corp.supernews.com...

"Myron Samila" <myronx19@no.spam.sympatico.ca> wrote in message
news:qRJbc.31538$j57.1436862@news20.bellglobal.com...
"Watson A.Name "Watt Sun - the Dark Remover"" <NOSPAM@dslextreme.com
wrote in message
news:c4kdn3$6i9sg$1@hades.csu.net...
I agree with their findings, especially page 4, paragraph 5 which
gives
white LEDs a much shorter lifetime rating than the generally
asserted
100,000 hours. The white LEDs I put on a PC board before xmas and
left
running continuously have dimmed to being all but worthless as
illumination.


http://www.lrc.rpi.edu/programs/solidstate/pdf/SPIE3938-39_Narendran.pdf

Where they of good quality? Osram claims 30,000 hours from their
white LEDs. They do
last longer, but the white component only lasts 30k hours.

Yeah, that's the problem. The phosphor dims, just like the phosphor in
a CRT when you have a pattern constantly on the screen in the same
place. The bright spots get burned in, so you can read it even when you
turn the monitor off. The same thing happens to the phosphor in the
LED, it dims, and then there's little blue and almost no white.

THe LEDs I used were from Hong Kong, and no, they were cheap, so gauging
by the price they were low quality.

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19

 

[repatch]

"John Fields" <jfields@austininstruments.com> wrote in message
news:jf5170dtkb8nrq63lnjjtjihaemck2murk@4ax.com...

On Sun, 4 Apr 2004 17:34:09 -0500, "D. Weatherwax" <davew@joink.com
wrote:

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't
be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

---
The white LED's I have on hand have a water-clear plastic T1-3/4
housing, and when you look through it into the reflector cup (when
it's not illluminated) you see a light yellow non-reflective coating
on it, which glows bright white when the LED is on. The chip itself
doesn't seem to be emitting blue, so I believe that what's happening
is that it's generating UV which is being used to pump the blend of
phosphors in the cup into emitting the three primary colors which we
then perceive as white light.

I doubt it. UV LEDs, while they exist, aren't yet "good enough" for
widespread use in white LEDs. The LED you have is almost certainly blue.
TTYL

 

[John Larkin]

On Sun, 04 Apr 2004 18:27:29 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 4 Apr 2004 17:34:09 -0500, "D. Weatherwax" <davew@joink.com
wrote:

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

---
The white LED's I have on hand have a water-clear plastic T1-3/4
housing, and when you look through it into the reflector cup (when
it's not illluminated) you see a light yellow non-reflective coating
on it, which glows bright white when the LED is on. The chip itself
doesn't seem to be emitting blue, so I believe that what's happening
is that it's generating UV which is being used to pump the blend of
phosphors in the cup into emitting the three primary colors which we
then perceive as white light.

The spectral curves usually show a big, sharp blue peak and a broader
yellow. If it's not too diffused, you can often shine the spot on a
piece of paper and see regions of blue and yellow, because the
phosphor geometry is generally sloppy.

John

 

[John Fields]

On Mon, 05 Apr 2004 08:54:19 -0700, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Sun, 04 Apr 2004 18:27:29 -0500, John Fields
jfields@austininstruments.com> wrote:

On Sun, 4 Apr 2004 17:34:09 -0500, "D. Weatherwax" <davew@joink.com
wrote:

Is this true? Do white LED's actually use phosphor to generate the white
light? I assumed it was doping in the silicon junction that produced
specific wavelengths of light just like red or green LED's. If phosphor
generates the white light, what wavelength excites the phosphor? It can't be
infrared (can it??). Do they make LED's that output in the UV region, and
then put phosphor in front of the junction?

---
The white LED's I have on hand have a water-clear plastic T1-3/4
housing, and when you look through it into the reflector cup (when
it's not illluminated) you see a light yellow non-reflective coating
on it, which glows bright white when the LED is on. The chip itself
doesn't seem to be emitting blue, so I believe that what's happening
is that it's generating UV which is being used to pump the blend of
phosphors in the cup into emitting the three primary colors which we
then perceive as white light.


The spectral curves usually show a big, sharp blue peak and a broader
yellow. If it's not too diffused, you can often shine the spot on a
piece of paper and see regions of blue and yellow, because the
phosphor geometry is generally sloppy.

---
I'll check it out; thanks, John.

--
John Fields